1. Field of Invention
The present invention relates to a light guide plate which is supplied with light sideways and deflects the light to output from an emission face, further relating to a surface light source device employing the light guide plate, still further relating to a liquid crystal display employing the surface light source device for lighting of LCD panel, in particular, front-lighting of it.
2. Related Art
A surface light source device of a type comprises a light guide plate having an end face, through which light is introduced, and two major faces (i.e. faces larger than end faces) one of which provides an emission face, being employed for various uses such as back-lighting or front-lighting for a liquid crystal display. Basic performance of surface light source devices of such a type greatly depends on light guide plates employed therein.
A basic function of a light guide plate is to change a propagation direction (roughly in parallel with an emission face of the light guide plate) of light introduced into the light guide plate through a side end face so that the light is emitted from the emission face. As known well, a simply transparent light guide plate to which no modification is applied is capable of deflecting light slightly, providing an unsatisfactory brightness. Therefor any means for promoting emission from the emission face is required.
Means for promoting emission from a light guide plate relies upon one of the followings or some of them as combined.
(1) Scattering power within a light guide plate (light scattering guide plate);
(2) Emission face (a major face) provided with light diffusibility;
(3) Back face provided with light diffusibility;
(4) Emission face provided with light-refractive unevenness;
(5) Back face provided with light-refractive unevenness.
Ways based on (1) provide uniform and highly effective emission with ease. However, the emission is subject to have a preferential direction much inclined with respect to a frontal direction. (Usually, the inclination is about 60 to 75 degrees to a normal with respect to the emission face.) Therefore, a member (prism sheet) for modifying the inclined direction to the frontal direction must be arranged. Although employment of a light diffusion sheet brings some increase in frontal emission, it involves a wide light diffusion which leads to reduction in light energy efficiency.
Ways based on (2) or (3) hardly provide uniform and effective emission. The emission is also preferentially directed to oblique directions as in the case of (1). An increased light diffusibility checks the efficiency because of factors such as wide range scattering or absorption by light scattering elements (e.g. white ink).
Ways based on (4) are capable of causing light to escape from the emission face with ease while positive direction conversions are less effected. Accordingly, it is hardly expected to realize a highly efficient emission. In particular, it is not advantageous that they fail to generate light which travels from the back face to the emission face.
Ways based on (5) positively generate light which travels from a back face to an emission face of a light guide plate, being free from wide range light scattering. Accordingly, the ways are latently capable of efficiently generating an emission directed to approximately frontal directions. A further merit is that a good applicability to a front-lighting-type LCD, which has been used often recently, is realized.
However, in practice, prior arts fail to control propagating direction of emission sufficiently.
FIG. 1a to FIG. 1c illustrate examples to which the above (5) is applied. Referring to the illustrations, reference number 1 indicates a light guide plate made of a transparent material such as acrylic resin, which has a side end face to provide an incidence end face 2. A primary light source L is disposed beside the incidence end face 2 to be supplied with light from the primary light source L. One of two major faces 3, 4 of the light guide plate 1 provides an emission face 3. The other major face (called xe2x80x9cback facexe2x80x9d) is provided with a great number of recesses 5 with slopes 5a, 5b in profile.
The primary light source L emits light, which is introduced into the light guide plate 1 through the incidence end face 2. Upon encountering a recess, the propagation light within the light guide plate 1 (as represented by G1, G2) is inner-reflected by one slopes 5a to be directed to the emission face 3. Inner-incidence angle is denoted by xcex8 and an emission derived from beams G1, G2 is denoted by G1xe2x80x2, G2xe2x80x2. In other words, the slope 5a, which is relatively near to the incidence end face 2 (or primary light source L) compared with the other slope 5b, provides an inner-reflection slope for direction conversion. This effect is sometimes called edge-lighting effect.
The recesses 5 are formed like dots or linear channels. As shown in FIGS. 1a to 1c, formation pitch d, depth h or slope inclination xcfx86 of the recesses 5 is varied depending on distance from the incidence end face 2. Such variations prevent brightness on the emission face 3 from varying depending on distance from the incidence end face 2.
However, prior arts as shown in FIGS. 1a to 1c are subject to the following problems.
1. There is a region which is located behind the slope 5b as viewed from the incidence end face 2 and is hardly supplied with light. Therefore, a reduced formation pitch d gives no increasing in direction conversion efficiency, with the result that the emission face 3 is apt to show an unevenness in brightness.
2. Direction control regarding in a plane parallel to the incidence end face is not sufficiently effected. For example, if travelling directions of G1, G2 shown in FIG. 1a are parallel to the emission face 3 but not perpendicular to the incidence end face 2, emitted light G1xe2x80x2, G2xe2x80x2 will diverge to the right and left as viewed from the incidence end face 2. There is a remarkable amount of light component which is not perpendicular to the incidence end face 2 in an actual light guide plate. Therefore, it is difficult to obtain an emission which is directed to a desirable spatial direction (regarding in both planes perpendicular and parallel to the incidence end face).
3. Leaking of light from the back face 4 occurs easily because direction conversion to produce light directed to the emission face 3 relies on a single reflection (slope 5a). In other words, a condition of total reflection is broken easily at the reflection for direction conversion. For example, if beams G1xe2x80x2, G2xe2x80x2 are to be directed toward a generally frontal direction, inner incidence angle is about 45 degrees. This value is roughly equal to the critical angle of an interface between acrylic resin, which is a typical material of a light guide plate, and air. Therefore, a remarkable part of light directed somewhat downward leaks through the slope 5a. 
The present inventor proposed a light guide plate having a back face provided with a great number of micro-reflectors shapes like projections as shown in FIG. 2b, and surface light source device/LCD employing the light guide plate, which were disclosed (PCT/JP00/00871;WO00/49432). FIG. 2a is a partially enlarged perspective view around a micro-reflector to illustrate light paths of an inner input light. Note that size of micro-reflector is exaggerated for the sake of explanation.
As shown in FIG. 2a, a light guide plate 100 has a back face 114 provided with micro-reflectors 120 projecting from a general plane of the back face. The illustrated micro-reflector 20 has a shape like a block having six faces 121, 122, 123, 124, 127 and 128.
The faces 121 and 122 provide a guiding portion to effect a smooth light input for direction-conversion. The faces 121 and 122 meet each other at a ridge portion 126. On the other hand, the faces 123 and 124 effect reflections twice for direction-conversion, producing an inner output light. The faces 123 and 124 meet each other at a ridge 125.
The faces 27 and 28 are side walls limiting width of the micro-reflector. Orientation of each micro-reflector is represented by an extending direction of the ridge 125.
An orientation of the micro-reflector 120 is aligned to a main light input direction (coming direction), thereby making an approximately maximized light input to the micro-reflector and accordingly effecting an approximately maximized light direction conversion.
In FIG. 2a, an input light is represented by beams H1, H2 which is directed to a direction approximately perpendicular to the incidence face 12. However, light that is actually inputted into the micro-reflector 120 is not precisely parallel to the general plane of the back face 114 but progresses somewhat downward. Light that progresses precisely parallel to the general plane of the back face 114 or approaches the emission face 113 advances deep without being inputted to micro-reflectors 120. That is, the micro-reflectors 20 do not obstruct light advancing and make no region little light reaches, thereby effecting contrary to the recesses (See FIG. 1).
Viewing from the standpoint of the beams H1 and H2, the reflection faces 123 and 124 form a valley. The ridge 25 corresponds to a bottom of the valley. The valley gets narrower and shallower gradually. Therefore, beams H1 and H2 entering the valley are inner-reflected by one of the slopes 123 and 124, and then inner-reflected again by the other slope 124 or 123 almost without fail.
As a result, a light propagation direction is converted twice to produce inner output light J1, J2 directed to the emission face 113. Direction of the inner output light J1, J2 can be controlled within a remarkable range through adjusting directions (spatial directions) of slopes 123 and 124. If the directions of the slopes 123 and 124 of all micro-reflectors are adjusted so that every inner output light J1, J2, which is made from the input light H1, H2 in accordance with a main light coming direction, has a direction that generally accords with a normal with respect to the emission face 113, almost the whole of the emission face 113 provides an output light like a parallel flux directed to a generally frontal direction.
However, the above-improved light guide plate or surface light source device employing it remains a problem unsolved and required to be overcome. This problem is that they bring display contrast and resolving characteristics which are somewhat unsatisfactory if applied to a liquid crystal display of front-lighting type. This situation is illustrated in FIG. 3.
As known well, light is supplied to an LCD panel through an emission face of a surface light source device in an front-lighting arrangement, returning to the light guide plate with an intensity distribution according to the information to be displayed. Such returning light is emitted through a back face of the light guide plate while some of the returning light is incident to a micro-reflector from its just facade direction. This is shown by references R1, R2 in FIG. 3.
Returning light R1, R2 to a micro-reflector 120 is inner-incident to the micro-reflector 120 at positions which are almost always located on any slope (for example, on the slopes 121 and 122 as shown in FIG. 3). As a result, the returning light R1, R2 is subject to bent-emission involving deflection as shown by references S1, S2. Needless to say, such a bent-emitted light gives a reduced display contrast and leads to a reduction in sharpness of information to be displayed (i.e. blurring of an image to be displayed).
The present invention aims to overcome the above-mentioned problem s of prior arts. That is, an object of the present invention is to improve an light guide plate used for emitting light, which is introduced from a side end face (incidence end face), from an emission face so as to have no region which light is hard to reach and to have an emission direction easily controllable and further to have a heightened applicability to a front-lighting arrangement.
Another object of the present invention is provide a surface light source device which is capable of efficiently providing an illumination light that has a direction regarding well-controlled in both planes perpendicular and parallel to an incidence end face by means of said improved light guide plate without a particular need of direction modifying member such as prism sheet.
Still another object of the present invention is provide a liquid crystal display that allows an easy observation from a desired direction by applying said surface light source device to a backlight or front-lighting arrangement for the liquid crystal display. The present invention also aims prevent the front-lighting arrangement from bringing reduced contrast and sharpness of an image to be displayed.
The present invention resolves the problems based on an idea that direction conversion is effected by a great number of micro-reflectors like tablelands for effecting double inner-reflection which are formed on a back face of a light guide plate, wherein each of the micro-reflector has a pair of slopes adjacent to the tableland top face.
In the first place, the present invention improves a light guide plate that comprises two major faces to provide an emission face and a back face and a side end face for introducing light. The back face of the light guide plate in accordance with the present invention is provided with a great number of micro-reflectors for light-direction-conversion. Each of the micro-reflectors is a projection shaped like a tableland projecting from a general plane on which the back face generally extends, each of the micro-reflectors providing inside a flat bottom face and a valley adjacent to the bottom face.
The bottom face extends approximately in parallel with the general plane on which the back face generally extends and, the valley including first and second slopes and is formed as to tend to get narrower and shallower with an increasing distance from the bottom face.
This causes an inner input light reaching the valley to be inner-reflected by one of the first and second slopes and then inner-reflected by the other, causing an inner output light directed to the emission face to be produced. An extending directions of the valley may vary depending on position on the back face.
In the next place, the present invention improves a surface light source device that comprises at least one primary light source and a light guide plate having two major faces to provide an emission face and a back face and a side end face for introducing light from the primary light source.
In a surface light source device in accordance with the present invention, the back face of the light guide plate is provided with a great number of micro-reflectors for light-direction-conversion. Each of the micro-reflectors is a projection shaped like a tableland projecting from a general plane on which the back face generally extends, each of the micro-reflectors providing inside a flat bottom face and a valley adjacent to the bottom face.
The bottom face extends approximately in parallel with the general plane on which the back face generally extends and, the valley including first and second slopes and is formed as to tend to get narrower and shallower with an increasing distance from the bottom face.
This causes an inner input light reaching the valley to be inner-reflected by one of the first and second slopes and then inner-reflected by the other, causing an inner output light directed to the emission face to be produced. An extending directions of the valley may vary depending on position on the back face.
Light introduction may be done from a plurality of directions different from one another. In this case, said great number of micro-reflectors are preferably classified regarding orientation into groups corresponding to said plurality of directions respectively so that micro-reflectors belonging to each of said groups has charge of a partial production of said inner output light according to a share allotted to each of said groups.
Such an improved surface light source device may be applied to a lighting arrangement of a liquid crystal display. In particular, if the surface light source device is applied to a front-lighting arrangement of a liquid crystal display of front-lighting type, the surface light source device brings a merit that neither display contrast nor sharpness is spoiled because a reduced bent-emission is produced by the returning light from the LCD panel (See FIG. 3 and related description).